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Premyrsinane-type diterpenoids have been considered to originate from the cyclization of a suitable 5,6- or 6,17-epoxylathyrane precursor. Their biological activities have not been sufficiently explored to date, so the development of synthetic or microbial approaches for the preparation of new derivatives would be desirable. Epoxyboetirane A (<b>4</b>) is an 6,17-epoxylathyrane isolated from <i>Euphorbia boetica</i> in a large enough amount to be used in semi-synthesis. Transannular cyclization of <b>4</b> mediated by Cp₂Ti^IIICl afforded premyrsinane <b>5</b> in good yield as an only diasteroisomer. To enhance the structural diversity of premyrsinanes so their potential use in neurodegenerative disorders could be explored, compound <b>5</b> was biotransformed by <i>Mucor circinelloides</i> NRRL3631 to give rise to hydroxylated derivatives at non-activated carbons (<b>6</b>–<b>7</b>), all of which were reported here for the first time. The structures and absolute configurations of all compounds were determined through extensive NMR and HRESIMS spectroscopic studies.